In general, an optical communication system includes a line monitor for sampling and monitoring a part of transmission signal in order to control the power of the transmission signal and detect errors in the communication. A two-way line monitor is used in a two-way optical communication system also to monitor a reflected light.
According to some conventional two-way line monitors, there are disadvantages in that it is difficult to sense the transmission light precisely and the monitor may be complicated in structure, for example, two beam splitters are required to sense lights.